Optical pickups in CD, DVD, and newer BD drives have gratings (called holographic optical elements, HOES) in them to diffract the laser beam for various purposes, such as focus tracking and signal detection. The gratings can be straight to diffract an incident beam into several orders, or curved to have additional functions other than splitting the beams, such as focusing the beams to the detectors.
One way of mass producing the gratings is to make a mold first, and manufacture the gratings by press molding. For binary gratings, photolithographic processes have been used to make the grating molds. However, as the grating design evolves from binary to grayscale (blazed), and the grating period becomes smaller for shorter wavelengths, photolithographic processes become less suitable for producing the required grating molds.
It is known that single point diamond turning can be used to machine gratings and grating molds. In the known process, the diamond tool has a single sharp tip. The diamond tool makes one or more ruling passes within one period to remove material to fabricate the grating or grating mold. This process works well for simple grating shapes.
Advanced grating designs for optical pickup can require more complicated grating profiles within one period to achieve desired functions of the grating. As an example, a particular design for an optical pickup grating has the following shape shown in FIG. 1. Such gratings have fine, micrometer- to sub-micrometer-size features that are difficult to machine with a simple, single-point diamond tool. What is needed is a way to make structured diamond tools that can machine gratings or grating molds with complicated shapes. The present invention fulfills this need.